1. Field of the Invention
The present invention relates to techniques for cophasing the elements of a transmitting or receiving phased array antenna and, more particularly, to techniques for cophasing the elements of a phased array antenna wherein to determine the phase setting between any two feed elements, an upper and a lower sideband signal are concurrently provided at a first one and a second one of the feed elements, respectively, and next at the first one and a third one of the feed elements, respectively, to permit the estimated phase angle between the concurrent sideband signals to be measured and then compared with each other to determine the phase setting to be used for the third one of the feed elements with respect to the second one of the feed elements.
2. Description of the Prior Art
Phased antenna arrays having directive characteristics are well known in the art and are preferred for certain applications over omni-directional antennas since a directional antenna provides greater signal-to-noise ratio and, of course, better gain. However, to provide a beam of electromagnetic energy in a certain direction, the individual elements of the phased array must be properly cophased. One technique for cophasing the elements for a particular directional wavefront is to compute the expected phasings and then store such phasings in the phase controller associated with the array for subsequent use. Once computed and stored, such phase settings are not changeable and are used regardless of conditional changes which may occur until new phase settings are computed and stored.
Typical prior art techniques for providing a more active rather than passive cophasing scheme are disclosed in, for example, the following references. U.S. Pat. No. 3,140,490 issued to W. Sichak et al on July 7, 1964 relates to a communication system with automatic antenna beam steering wherein the induced signal at one antenna is modulated with a constant frequency signal and is compared with the original modulating signal so that when an out-of-phase condition is noted a control signal is generated to cause a phase shift in the first antenna signal to produce an in-phase condition.
U.S. Pat. No. 3,271,770 issued to A. B. Lees on Sept. 6, 1966 relates to an antenna phasing control system for adjusting the relative phasing of electrical energy applied to individual antennas in an array in accordance with the phase conjugate of the signals received by the individual antennas. A control signal is generated to alter the relative phasing of a master oscillator signal to place the radiated signal at the associated antenna in phase conjugate with the received signal.
U.S. Pat. No. 3,378,846 issued to O. Lowenschuss on Apr. 16, 1968 relates to method and apparatus for testing and aligning each of N elements in a phased array antenna. More particularly, each element has a separate phase shifter associated therewith which phase shifter is individually controlled by B control bits from a common command circuit. By checking the amplitude and phase changes of the received signal it can be determined if each element is operable and aligned and appropriate remedies applied if the element is inoperable or misaligned.
U.S. Pat. No. 3,453,623 issued to W. T. Blackband et al on July 1, 1969 relates to a phased-optimized antenna system comprising two aerials having uneven polar sensitivity characteristics. There, a phase sensitive detector receives separate signals from a perturbation oscillator and the perturbation signal from a common signal path and generates output control signals to control the individual phase shifters and bring the signals associated with the two aerials into phase with each other.
The problem remaining in the prior art, however, is to provide a technique for permitting the phase of each element of a phased antenna array to be accurately and actively determined to permit a planar wavefront to be launched to or received from a distant location by such antenna and to be easily maintained despite possible changes in the phase setting requirements of the individual elements of the array due to thermal and aging effects.